Method for removing ammonia and carbon dioxide gases from a steam

ABSTRACT

A method for reducing residual ammonia gas from a steam in a steam condenser. The method includes forming a condensate and a residual gas in a first stage, then forming a condensate curtain in the second stage and passing the residual gas through the condensate curtain prior to exiting the steam condenser. The method may be employed to remove ammonia and carbon dioxide gas from a geothermal steam.

The present application is a divisional of U.S. Ser. No. 08/634,241filed Apr. 18, 1996 and now U.S. Pat. No. 5,772,709.

FIELD OF THE INVENTION

The present invention relates to removing contaminant gases from asteam, and more particularly to removing ammonia and carbon dioxide froma steam in a two stage system having a first stage condenser forcreating a substantially predetermined residual gas composition andcondensate in the first stage and a second stage column for absorbingammonia and carbon dioxide gases into the condensate from the residualgas composition.

BACKGROUND OF THE INVENTION

Condensers are often connected to a low pressure exhaust of a steamturbine (1) to produce a vacuum or desired back pressure for improvementof the power cycle efficiency; and in some circumstances (2) to condensethe turbine exhaust steam for possible reuse; and (3) to deaerate thecondensate.

Traditional power generation facilities have employed coal, gas, naturalgas, oil or other burnable materials as a source of power. In thesesystems, a quantity of water is turned to steam and passed through aturbine, condensed and reexposed to the heat source.

As the awareness of energy sources and their impact on the environmenthas increased, the demand for alternate power sources has risen. Whilegeothermal steam can offer significant advantages for power generation,the steam may contain contaminants such as potentially harmful gaseswhich are undesirable in the atmosphere. In fact, the introduction ofsome contaminants often found in geothermal steam are restricted bygovernment regulation. Additionally, the removal of gases from thegeothermal steam is beneficial to a reduction in the utilityrequirements for operating the power system.

Therefore, the need exists for a condenser for use with contaminatedsteam which provides for reduced emission of selected contaminants,thereby creating a residual gas mixture of substantially predeterminedcomposition. The need also exists for a method of selecting suchcondensers to adequately remove the selected contaminants and produce acontrollable condensate. A further need exists for subsequently treatingthe residual gas to absorb ammonia and carbon dioxide from the residualgas into the condensate.

SUMMARY OF THE INVENTION

The present invention includes a method for removing ammonia and carbondioxide from a steam in a two stage steam condenser assembly. In thefirst stage, a residual gas of a substantially predetermined compositionand condensate are formed in a selected condenser. In the second stagethe residual gas is treated to reduce ammonia and carbon dioxide gasesby absorption into the condensate. As used herein, the term "residual"includes those gases that remain in a gaseous state after exposure ofthe geothermal steam to the condensing surfaces in the condenser. Thatis, the residual gases are unabsorbed into the condensate in thecondenser.

The present gas removal process includes absorption of the ammonia andcarbon dioxide gases into a condensate; and a subsequent reaction of theabsorbed gases to form aqueous ammonium carbonate, thus reducing theamount of both ammonia and carbon dioxide to be vented from thecondenser assembly.

The first stage of the condenser assembly includes a condenser having avapor inlet, a condensing surface, a flow control surface, a subcoolingsection to allow a condensate to be cooled to a temperature below asaturation temperature in the condenser; and the second stage includes acolumn connected to the condenser, the column including a vapor outletand a condensate outlet, and a flow forming surface for configuring aflow of the condensate from the condenser to form a condensate curtainin the column, the curtain being disposed intermediate the vapor inletand the vapor outlet.

The present method for reducing ammonia and carbon dioxide gases in asteam following condensation in the condenser includes forming a curtainof temperature conditioned condensate and passing the residual ammoniagas through the conditioned water curtain to absorb at least a portionof the residual ammonia and carbon dioxide gases before exiting thecondenser assembly.

The method for selecting a first stage steam condenser to produce acondensate and residual gas composition of substantially predeterminedamounts of ammonia and carbon dioxide gases includes specifying an inletamount of water vapor, ammonia, carbon dioxide and a noncondensable gas;estimating an amount of the ammonia and carbon dioxide unabsorbed by acondensate of the water vapor produced by condensation with a condensertube in the steam condenser; determining a corresponding partialpressure of the ammonia and carbon dioxide gas; calculating the partialpressure of the noncondensable gas based upon the partial pressures ofthe water vapor, ammonia, carbon dioxide and a total specified pressurein the steam condenser; calculating the amount of noncondensable gas andcomparing the calculated amount to the specified amount; and reiteratingthe previous steps until the calculated amount of the noncondensable gasequals the specified amount. Once the final composition of the gasesexiting the condenser have been determined, the condenser sizing isselected with corresponding flow control surfaces and cooling tubes toachieve the predetermined conditions. The residual gas mixture thusformed by the first stage condenser is then treated in a column bypassing the residual gas through a condensate curtain to absorbadditional ammonia and carbon dioxide gases in the condensate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross sectional side elevational view of thegeothermal steam condenser.

FIG. 2 is a side elevational cross sectional view of the column and flowcontrol surfaces of the geothermal steam condenser.

FIG. 3 is an enlarged cross sectional side elevational view of the vaporinlet to the condenser and the upper portion of the column.

FIG. 4 is an enlarged cross sectional side elevational view of the flowcontrol surfaces in the condenser body.

FIG. 5 is an enlarged cross sectional side elevational view of thecurtain forming surfaces in the column.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, the two stage steam condenser assembly 10 of thepresent invention includes a first stage condenser housing 20 and asecond stage forming column 60. It is understood the present inventionmay be employed with a geothermal steam or any other steam havingcontaminants.

The condenser housing 20 of the first stage is sized to retain aplurality of cooling tubes 22. The cooling tubes 22 are fluidlyconnected to a cooling water inlet 24 and a cooling water outlet 26.Tube support sheets 42 are disposed in the condenser housing to retainthe cooling tubes 22 and to direct the flow of vapor (steam) within thecondenser housing 20. Baffles 34 may also be used to direct the steamflow in the condenser housing 20.

The condenser housing 20 includes a vapor inlet 28 and an outlet 30. Thevapor inlet 28 is connected to a steam source such as a turbine exhaust.The outlet 30 is sized to pass condensate and uncondensed and unabsorbedgases from the condenser housing 20. In the present embodiment, theoutlet 30 has a circular periphery.

The condenser housing 20 also includes flow control surfaces 36 and 38.Referring to FIGS. 1-4, the flow control surface 36 is a weir forsetting an amount of condensate thermally coupled to the cooling tubes22. That is, the weir 36 determines the quantity of cooling tubes 22immersed in the condensate. The flow control surface 38 is a weir ormixing barrier for directing a surface flow of condensate to descendinto the condensate and further mix.

As shown in FIGS. 1-4, in the present embodiment, the cooling tubes 22are linearly arranged parallel to a longitudinal axis of the condenserhousing 20. In this embodiment, the weirs 36, 38 also function ascooling tube supports and extend perpendicular to the length of thetubes 22 and condenser housing 20. That is, the cooling tubes 22 passperpendicularly through the weirs 36, 38. For clarity, only two coolingtubes 22 are shown in the condenser housing 20 and extend between therespective weirs 36, 38. It is understood, the number and location ofthe cooling tubes 22 is greater than the two shown.

Referring to FIG. 2, the column 60 of the second stage is connected tothe outlet 30 of the first stage condenser housing 20 to permit thecondensate to flow from the housing into the column. The column 60includes a spillway 62; a forming chute 70; a curtain portion 80; acondensate outlet 82 and a vapor outlet 84.

Preferably, the column 60 is a cylindrical member with a substantiallycircular cross section. The spillway 62 extends from the junction withthe condenser housing 20 to the forming chute 70 and representsapproximately one-third the height of the column. As shown in FIGS. 2and 3, the forming chute 70 includes three surfaces: a restrictingsurface 72, a curtain forming surface 74 and a guiding surface 78. Therestricting surface 72 is a frustoconical or funnel shaped member,tapering from a larger diameter adjacent the spillway 62 to a narrowerdiameter at the curtain forming surface 74. The curtain forming surface74 is generally saw-toothed to define a plurality of discontinuities 75.Although the curtain forming surface 74 is generally saw-toothed, thesurface may be a substantially planar surface. The curtain formingsurface 74 extends above a lower edge of the restricting surface 72 toform a generally annular cavity therebetween.

The guiding surface 78 is a cylindrical surface extending below thecurtain forming surface 74. The forming chute 70 thus tapers thecondensate flow from the diameter of the column to a reduced diameter.

In one example of the present embodiment, the column 60 has a 16 inchdiameter, and an overall length of approximately 100 inches, where thespillway 62 has a length of 27 inches, the forming chute 74 has a lengthof 17.5 inches and the curtain portion 78 has a length of 55.5 inches.

The condensate outlet 82 is located in the bottom of the column 60 forallowing passage of condensate from the column. The vapor outlet 84extends from the side of the column 60 adjacent the curtain formingsurface 74. The vapor outlet 84 is connected to a vacuum source fordrawing uncondensed and unabsorbed water vapor and gases and through thevapor outlet.

OPERATION

In operation, cooling water is introduced to the cooling water inlet 24and passes through the cooling tubes 22 to exit through the coolingwater outlet 26. Steam, such as geothermal steam, enters the condenserhousing 20 of the first stage through the vapor inlet 28 and is directedby the baffle 34 to contact the outside surface of cooling tubes 22. Acondensate 12 is formed and the surface of the condensate 12 is shown asa dashed line in FIGS. 1 and 2. The condensate accumulates untilovercoming the weir 36 to descend through the outlet 30 and into thespillway 62 of the second stage column 60.

The descending condensate contacts the restricting surface 72 and beginsto form a cylindrical body. The condensate then passes over the curtainforming surface 74 to form a substantially continuous flow. Thecontinuous flow contacts the guiding surface 78 and follows the surfaceto its terminal end. A continuous curtain of condensate is thus formedand descends into the curtain portion 80. The condensate curtaindescends through the curtain portion 80 of the column 60 to accumulateadjacent the condensate outlet 82.

As the steam is condensed in the main condenser 20, a substantiallypredetermined portion of the ammonia and carbon dioxide is not absorbed.The residual gases passing from the condenser housing 20 may include anunacceptable or undesired level of uncondensed and/or unabsorbed gasessuch as ammonia and carbon dioxide. The residual gases are treated inthe second stage column 60 to remove additional amounts of ammonia andcarbon dioxide gases. As a vacuum is placed on the vapor outlet 84 inthe column 60, the residual gases, including ammonia and carbon dioxide,are drawn from the first stage housing 20 through the outlet 30 and intothe spillway 62. As a curtain of condensate is descending from theforming chute 70, the exiting vapor becomes "trapped" within thedescending condensate curtain. The relatively low pressure of the vaporoutlet 84 creates a pressure differential across the descending curtainand the residual gas mixture including ammonia and carbon dioxide areforced to pass through the descending curtain. As these gases passthrough the curtain, the ammonia and carbon dioxide gases experience anincreased residence time with the condensate 12, thereby causing aportion of the residual ammonia and carbon dioxide to be furtherabsorbed in the condensate.

Turning to the embodiment of FIG. 6, the condensate 12 descends from thespillway 62 and collects on the liquid distribution plate 92 to form ahorizontal curtain of condensate. The residual ammonia and carbondioxide gases are drawn through the curtain on the liquid distributionplate 92. Upon entering the packing they experience further residencetime and increased contact area with the condensate in the packing 96 tofurther absorb ammonia and carbon dioxide gases in the condensate.

By selectively controlling the parameters of the first stage condenser20 such as the number and disposition of cooling tubes 22, and thecondensate subcooling temperature, the amount of residual (uncondensedand/or unabsorbed gases), such as ammonia and carbon dioxide vaporexiting the first stage condenser 20 are controlled to predeterminedlevels. The temperature of the condensate 12 (and hence the condensatecurtain) can be adjusted by controlling the exposure of the condensate12 to the cooling tubes 22, that is, the condensate 12 may be subcooledbelow the condensing temperature. The amount of subcooling in turn iscontrolled by the temperature of the cooling water, and the depth of thecondensate about the cooling tubes 22. The depth of the condensate 12contacting the cooling tubes 22 is set by the weir 36.

SELECTING THE FIRST STAGE GEOTHERMAL STEAM CONDENSER

To select the first stage steam condenser 10, for the effective creationof a predetermined residual gas composition including ammonia and carbondioxide, a multi step process is employed. Specifically:

1. Inlet amounts of water vapor (steam), ammonia, carbon dioxide, andair (or other noncondensable gas) is known along with the operatingpressure and temperature of the condenser. These inlet amounts may bedetermined by measuring the actual steam components at the site, or bebased upon given values. The operating temperature and pressure is inpart determined by the geothermal steam parameters, the cooling waterparameters and the condenser parameters.

2. An amount of ammonia and carbon dioxide not being absorbed by thecondensate is estimated. Based upon equilibrium charts, such as those byW. VanKrevelen et al., the partial pressures corresponding to theestimated amounts of unabsorbed ammonia and carbon dioxide aredetermined in view of the operating temperature and pressure of thecondenser.

3. The partial pressure of the remaining water vapor is taken fromstandard steam tables at the operating temperature of the condenser.

4. As the partial pressure of the water vapor is known, and the partialpressure of the ammonia and carbon dioxide is known (based upon theestimated amount unabsorbed in the condensate) and the operatingpressure of the condenser has been identified, the partial pressure ofthe noncondensable gas is calculated.

5. Employing the general relationship between the partial pressure ofeach component and the moles of that gas (Pi/Mi), the accuracy of theinitial estimate of the ammonia and carbon dioxide gas not beingabsorbed in the condensate is determined.

6. An iterative process is used to calculate the final composition ofthe gas mixture and these steps are reiterated until the calculatedamount of the noncondensable gas equals the specified amount as setforth in step 1.

7. Once the calculated amount and the specified amount of noncondensablegas are equal, or within an acceptable degree of equivalence, acondenser is selected with corresponding flow control surfaces andcooling tubes to achieve the degree of absorption calculated and hence aresidual gas mixture of substantially predetermined composition.

Therefore, the present invention allows the selecting of a first stageof a steam condenser to produce a flow rate of a condensate having asubstantially predetermined composition along with a residual gasmixture of predetermined composition from a steam including (a)specifying an inlet amount of water vapor, a first gas and a secondnoncondensable gas; (b) estimating an amount of the first gas unabsorbedby a condensate of the water vapor; (c) determining a correspondingpartial pressure of the first gas; (d) calculating the partial pressureof the second noncondensable gas based upon the partial pressures of thewater vapor, the first gas and the total specified pressure in the steamcondenser; (e) calculating the amount of the second noncondensable gasand comparing the calculated amount to the specified amount; and (f)reiterating steps b-e until the calculated amount of the secondnoncondensable gas substantially equals the specified amount or iswithin an acceptable degree of equivalence. A first stage condenser isthen selected having sufficient thermal transfer capabilities to achievethe calculated absorption and create a residual gas with the calculatedcomposition. Specifically, the condenser is selected with flow controlsurfaces and cooling tubes to attain the required heat transfer. Theformed residual gas mixture and condensate are then interfaced in thesecond stage column in order to further reduce the ammonia and carbondioxide gases exiting the condenser assembly.

It is understood that the present invention may be employed with anycombination of the features disclosed, or their equivalents. While apreferred embodiment of the invention has been shown and described withparticularity, it will be appreciated that various changes andmodifications may suggest themselves to one having ordinary skill in theart upon being apprised of the present invention. It is intended toencompass all such changes and modifications as fall within the scopeand spirit of the appended claims.

What is claimed is:
 1. A method of absorbing ammonia and carbon dioxidegases from a steam containing water vapor, ammonia and carbon dioxidegases, comprising:(a) introducing the steam containing water vapor,ammonia and carbon dioxide gas into a condenser through a vapor inlet;(b) condensing a portion of the steam to form a condensate and aresidual gas containing unabsorbed ammonia and carbon dioxide; (c)forming a continuous condensate curtain between the vapor inlet and avapor outlet; and (d) withdrawing the residual gas through thecondensate curtain prior to exiting the condenser.
 2. The method ofclaim 1, further comprising regulating the temperature of the condensateto increase absorption of the ammonia and carbon dioxide gases in thecondensate curtain.
 3. A method of identifying a first stage of steamcondenser to produce a substantially predetermined condensate and aresidual gas composition of ammonia and carbon dioxide from a steamcontaining water vapor, ammonia and carbon dioxide gases, comprising:(a)specifying an inlet amount of water vapor, ammonia, carbon dioxide and anoncondensable gas; (b) estimating an amount of the ammonia and carbondioxide gases unabsorbed by a condensate of the water vapor produced bycondensation with a condenser tube in the steam condenser; (c)determining a corresponding partial pressure of the ammonia and carbondioxide gases; (d) calculating the partial pressure of thenoncondensable gas based upon the partial pressures of the water vapor,the ammonia, the carbon dioxide and a total specified pressure in thesteam condenser; (e) calculating the amount of noncondensable gas andcomparing the calculated amount to the specified amount; and (f)reiterating steps b-e until the calculated amount of the noncondensablegas substantially equals the specified amount.
 4. A method of selectinga first stage in steam condenser for absorbing a predetermined amount ofammonia and carbon dioxide gases from a steam having an initial knownconcentration of a noncondensable gas, ammonia and carbon dioxide gases,comprising:(a) estimating the amount of ammonia and carbon dioxide thatwould be unabsorbed by a condensate of water vapor at a given operatingtemperature and pressure; (b) calculating the partial pressures of theunabsorbed ammonia and carbon dioxide in the steam; (c) calculating thepartial pressure of the noncondensable gas in the steam, based on thecalculated partial pressures of ammonia and carbon dioxide, the knownpartial pressure of the water vapor, and the total condenser pressure;(d) calculating the amount of noncondensable gas in the steam based onthe calculated partial pressure of the noncondensable gas; (e) comparingthe calculated amount of noncondensable gas to the known concentrationof noncondensable gas; (f) repeating steps (a-e) until the known amountand the calculated amount are substantially equal to a desired amount;and (g) selecting a steam condenser having sufficient condensateconditioning surfaces to absorb the ammonia as determined by step f. 5.A method of selecting a first stage in a steam condenser to produce acondensate and a substantially predetermined composition of a residualamount of a first gas from a steam, comprising:(a) specifying an inletamount of water vapor, a first gas and a second noncondensable gas; (b)estimating an amount of the first gas unabsorbed by a condensate of thewater vapor produced in the steam condenser; (c) determining acorresponding partial pressure of the first unabsorbed gas; (d)calculating the partial pressure of the second noncondensable gas basedupon the partial pressures of the water vapor, and the first unabsorbedgas, and a total specified pressure in the steam condenser; (e)calculating the amount of the second noncondensable gas and comparingthe calculated amount to the specified amount; and (f) reiterating stepsb-e until the calculated amount of the second noncondensable gassubstantially equals the specified amount of the second noncondensablegas.
 6. The method of claim 5, further comprising:(a) selecting a steamcondenser with a condensate subcooling section for conditioning thetemperature of the condensate to enhance absorption of a portion of thesecond noncondensable gas in a second stage of the condenser.
 7. Amethod of reducing the emission of a contaminant gas from a steamcontaining water vapor and the contaminant gas following condensation ofa portion of the steam in a steam condenser, comprising:(a) forming acontinuous curtain of condensate; and (b) passing the contaminant gasthrough the condensate curtain to absorb a portion of the contaminantgas before exiting the steam condenser.
 8. The method of claim 7,further comprising lowering the temperature of the condensate curtain toincrease the absorption of the contaminant gas in the condensatecurtain.
 9. The method of claim 8, wherein lowering the temperature ofthe condensate curtain includes subcooling a portion of the condensedsteam.
 10. The method of claim 7, further comprising forming asubstantially closed loop curtain of the condensate to enhanceabsorption of the contaminant gas.
 11. The method of claim 7, furthercomprising establishing a temperature of the condensate to enhanceabsorption of the continuant gas in the condensate curtain.
 12. A twostage process for removing a contaminant gas from a steam,comprising:(a) introducing the steam and the contaminant gas into acondenser to form a condensate and a residual gas mixture, the residualgas mixture including the contaminant gas; and (b) passing the residualgas mixture through a continuous condensate sheet to absorb a portion ofthe contaminant gas into the condensate sheet.
 13. A process forremoving a gas from a steam, comprising:(a) occluding a passage in acondenser from a vapor inlet to a vapor outlet by a continuouscondensate curtain; (b) introducing the steam and the gas into the vaporinlet to form the condensate curtain and a residual gas mixture, theresidual gas mixture including the gas; and (c) passing at least aportion of the residual gas mixture through the condensate curtain toabsorb a portion of the gas into the condensate curtain.